This disclosure relates to a ball screw actuator, and more specifically, to an electromechanical actuator for aerospace applications.
One type of electromechanical actuator (EMA) uses a ball screw driven by an electric motor, and optionally through a gearbox. When designing small, high power density EMAs, the rotational inertia exerted on the ball screw by the motor can be problematic. A large motor will produce a significant amount of rotational inertia. When a gearbox is used, the inertia of the motor imparted to the ball screw is proportional to the motor's inertia multiplied by the gear reduction ratio squared.
The inertia of the motor is important when sizing the gear train, the ball screw, and/or the support structure. A typical EMA includes one or more end stops to limit actuator travel at fully retracted and/or fully extended positions. If the traveling actuator impacts its end of travel stop, the rotational inertia of the motor will tend to cause the actuator to continue driving through the end stop, causing significant damage to the EMA. If the end stops are strong enough to maintain their integrity, the next weakest link, typically the ball screw or the gearbox, can be damaged.
Historically, damage to the EMA is avoided by over-designing the gearbox, the stops and surrounding support structure to handle the intense torque spike associated with the nearly instantaneous stopping of the ball screw as the ball screw impacts its end of travel stop, and the motor exerts its rotational inertia. As the motor continues to rotate with the ball screw stopped, the internal shafting, gears, and support structure distort. Over-designing the EMA to handle this torque spike results in an actuator that is significantly larger and heavier than it would otherwise have to be.